The invention relates to compositions which can be converted into glass ribbon. The glass compositions according to the invention are more particularly intended for applications of the aeronautical-window type, without correspondingly being limited to such applications.
With regard to aeronautical-type applications, the invention is intended more particularly for windows which, after chemical toughening, may exhibit high compressive stresses over a great depth.
For these applications, especially aircraft or helicopter windows, the requirements in terms of mechanical strength are such that a strengthening operation is generally carried out by chemical means rather than simply by thermal means, as is usual, for example, for motor-vehicle windows. Chemical toughening may also be used for other very demanding applications such as, for example, the windows for armoured, railway or maritime vehicles, or else for motor vehicles.
As in the case of thermal toughening, chemical toughening consists in putting the surface of the glass into compression, the rupture strength of the glass being increased by an amount that is more or less identical to the magnitude of the compressive surface stress generated by the treatment, in this case generated by substituting some of the alkali metal ions of the surface layers of the glass with other less bulky ions which will be inserted into the glassy network.
In the case of forces exerted over the entire window, for example the pressure exerted by the air in a pressurized cockpit, and in the case of more dynamic forces, for example a bird strike, the impact of which generates a very high force which may cause the glass to break starting from a face in extension exhibiting surface defects, the quality of the mechanical strengthening is dictated, on the one hand, by the value of the compressive surface stress and, on the other hand, by the treated depth. Ideally, the object of the chemical toughening operation is thus to place the surface layers of the glass article treated under a very high compressive stress over a very great depth, at least equal to the depth of the largest possible defect.
For a given glass composition, the exchanged depth depends on the duration of the ion-exchange treatment and on the temperature at which it is carried out. However, a rise in temperature results in an increase in the rate of relaxation of the stresses and consequently results in low rupture stresses. Likewise, extending the treatment for too long a period results in an unsatisfactory degree of toughening, the stresses then having the necessary time to relax.
These considerations have resulted in the development of novel glass compositions which are more favourable to ion exchange than conventional window glass compositions and, especially, allow greater exchange depths to be obtained for treatment times not exceeding a few hours. Thus, Patent Application FR-A-2,128,031 provides silica-soda glasses, using oxides commonly encountered in conventional industrial glasses, satisfying the following composition, defined by its percentages by weight:
SiO.sub.2 65.0-76.0% Al.sub.2 O.sub.3 1.5-5.0% MgO 4.0-8.0% CaO 0.0-4.5% Na.sub.2 O 10.0-12.0% K.sub.2 O 1.0-4.0% B.sub.2 O.sub.3 0.0-4.0%
these elements representing at least 96% by weight of the glass and furthermore satisfying the percentages by weight CaO/[CaO+MgO] between 0 and 0.45, and K.sub.2 O/[Na.sub.2 +K2O] between 0.05 and 0.35, these limits being inclusive.
The compositions defined hereinabove make it possible, after 24 hours, to obtain a reinforcement depth which is from 1.8 to 3.3 times greater than the depth obtained with ordinary window glass. However, in Patent Application FR-A-2,128,031 the ion-exchange processes are relatively short, these being systematically limited to times of at most 24 hours, which allows the thickness of the reinforced layer to reach at most approximately 100 microns (for a 400.degree. C. treatment temperature). However, for aeronautical applications in particular, this thickness must be much greater, for example approximately 300 microns, which comes back to the problem mentioned earlier with conventional glass compositions.
It has also already been demonstrated, in Patent EP-0,665,822, that such glass compositions are also suitable for long treatments, typically of at least 72 hours and especially of more than 10 days or even more than 15 days--the treatments may in some cases exceed some twenty days--and can consequently be used to obtain glass articles strengthened by ion exchange to a great depth, for example 200 microns or more, while at the same time maintaining very satisfactory strengthening levels, for example with compressive surface stresses of at least 400 MPa. Thus, glass products, the composition of which satisfies the formula known from Patent FR-A-2,128,031, and which have undergone a strengthening treatment by ion exchange at a temperature such that the compressive surface stress is at least 400 MPa and preferably at least 500 MPa, for a treated depth of at least 200 microns, or alternatively articles whose compressive surface stress is at least 650 MPa, for a treated depth of at least 75 microns, have been described.
By way of indication, the treatment carried out may, for example, be for 18 days at a temperature of 415.degree. C., which results in compressive surface stresses of approximately 500 MPa, and an exchanged depth of approximately 265 microns. If the envisaged application can allow shallower treatment depths, it is also possible to obtain substantially greater strengthening levels with, for example, compressive surface stresses of approximately 700 MPa or higher, using low-temperature (for example 350.degree. C.) treatments for times which are more or less identical to the previous case, this time with a treatment depth of approximately 80 microns.
Although the mechanical properties obtained are satisfactory, it appears all the same that they require relatively long toughening treatment times.
Moreover, apart from the requirements regarding the mechanical properties, it is necessary, more particularly in the case of aeroplane windows, to obtain good chemical resistance and, especially, good hydrolytic resistance. This is because the consumption of glass for this type of product is relatively low compared with the production capacities of furnaces in the glass industry; it is therefore desirable to keep the glass for a time which may be up to several years, while maintaining the properties of the glass.